In recent years what is known as a flow fence has been incorporated in gas turbine engines to prevent compressor surge. Briefly, a flow fence consists of a split ring received in an annular recess in the compressor inlet housing of a gas turbine engine. The ring has free ends which are displaceable relative to one another between a fully expanded and fully contracted position. In the fully expanded position the ring offers no resistance to flow into the compressor. In the fully contracted position it blocks flow into the compressor and more particularly to the tips of the compressor blades, thereby preventing compressor surge during certain operating conditions.
A ring of this type usually is actuated by a control system which senses particular engine parameters whose magnitude indicates proximity to a surge condition and uses these parameters to control the ring between these two positions. This is effective in simply and efficiently eliminating stall.
One of the problems with this device, however, is that it is either on or off. The sudden change in the flow path through the compressor causes step changes in power that may be annoying to an operator. It is desirable then to slowly modulate the opening of the split ring to ease the transition and prevent compressor surge and a step change in power. However, with prior art split rings it has not been possible to maintain the inner diameter of the split ring concentric with the flow passage over the entire range of travel.
In U.S. Pat. No. 3,841,790 to Stein et al, this problem was partially overcome by inclusion of a split ring in a circumferential recess in the inlet housing. Adjustment means were connected between the housing and points adjacent to the free ends of the ring for displacing the free ends relative to one another through a predetermined path to vary the reference diameter of the ring. A guide means was connected between the housing and the ring at a point midway between the points on the ring and prevented circumferential movement but permitted radial movement so that the reference diameter of the ring was maintained substantially concentric relative to the inlet housing irrespective of the variation in the reference diameter of the ring.
One problem exhibited by the Stein et al construction was, even though adjustment between "full in" and "full out" could be accomplished gradually, surge margin still suffered during retraction at high speed.
Accordingly, it is an object of the invention to provide an improved flow fence which improves surge margins in a gas turbine engine.
It is another object of the invention to provide an improved flow fence which, upon insertion and withdrawal from the flow field, exhibits a gradual effect on the flow.